A Numerical Study on the Detailed Structure of Hydrogen/Air Bunsen Flame

Abstract

The detailed flow, temperature and concentration fields of a hydrogen-air Bunsen flame have been examined by a precise numerical study that uses the exact transport properties and the full chemical reaction mechanism. The study has revealed that the chemical reactions along the flame cone portion do not remain uniform; the H₂ consumption rate decreases towards downstream. The decrease of H₂ concentration of the mixture coming into the cone, caused by the radial outward diffusion of mobile H₂ molecules, leads to the slowdown of the main H₂ consuming elementary reactions downstream. Circumferential curvature of the axisymmetric flame and thermal diffusion are not important in the observed behavior. It is also revealed that the structure of the flame tip is fundamentally two-dimensional, in the sense that the mixture coming into the tip continues the combustion reaction with a strong aid of heat diffusion from the outer flame cone. The result raises a serious question if the tip can be called a part of the self-sustained premixed flame.